A cache and backing storage mechanism is responsible for managing the keys, reference and change (RC) bits required for storage protection purposes in computer systems. The IBM System/370 Extended Architecture Principles Of Operations, 1983, discusses the use of the keys and RC bits on pages 3-8 to 3-12.
Futhermore, IBM System Journal, v.3 no.2 1964 pages 144-164 outlines the logical structure of the the System 360, with specific description of the storage protection keys. Each program and each page in main storage is assigned a key. When a program attempts to access a page of storage its key is compared with the key assigned to that page; if the keys match then access is allowed. If a data fetch is allowed, then the page's reference bit is set at the time of access; if a data store is allowed, then the page's reference AND change bits are set at the time of access. All of these bits can be directly read and written through special instructions (KEY OPs) by the operating system.
Access to these Storage Protection (SP) bits needs to be very fast because fetches and stores of data are held off while the keys are checked. Therefore, past implementations have kept all of the SP bits required for every location of main storage in a large high speed array in the head of the system control element (SCE). This is an expensive proposition which uses up valuable real estate and has prevented the growth of main storage because of the lack of room in the SCE.